1. Field of the Invention
This invention relates to an undercoat composition for a powder coating of a heat-resistant resin on a metal substrate at a high coating temperature, in particular, it relates to an undercoat composition with excellent heat-resistance and excellent adhesiveness to both the upper resin layer and the metal substrate.
2. Description of the Prior Art
Various resins are coated on a metal substrate to improve the corrosion resistance of the metal. In this case, an undercoat composition is used for example, for prevention of dry corrosion of the metal and for improvement of adhesiveness between the metal and the upper resin layer.
Mainly, the undercoat composition must have excellent adhesion to both the metal and the resin.
Such an undercoat composition includes specific organic compounds such as epoxy resins, phenol resins, aminobismaleimides, etc. Although these organic compounds are heat-stable up to a certain temperature, the compounds show propensity to decompose on being kept at the temperature of 300.degree. C. or more. Accordingly, a resin with a high melting point cannot be powder-coated on the undercoat layer made of these compounds at a high coating temperature (300.degree. C. or more). For example, the melting points of tetrafluoroethylene-hexafluoropropylene copolymer (FEP) and perfluoroalkoxy resin (PFA) are 275.degree. C. and 310.degree. C., respectively. Thus, these resins must be heated at a higher temperature than their melting points when they are powder-coated on the undercoat layer. In this case, the organic compounds mentioned above that are contained in the undercoat layer decompose at such high temperatures. Therefore, the adhesiveness between the metal and the coating resin becomes inferior, or it is impossible to achieve powder coating because evolution of a gas from the decomposed compound brings about the foaming of the coated resin.
To solve this problem, the use of inorganic binders as the undercoat composition instead of these organic compounds is proposed. Inorganic binders for the coating of fluorocarbon resins on a metal substrate include, for example, fluorocarbon resins, chelate compounds of Group IV transition metals, and water-insoluble fine powdered refractories (disclosed in Japanese Patent Publication No. 55-39188); or fluorocarbon resins, phosphoric acid, inorganic peroxides, and oxo acids (disclosed in Japanese Patents Publication No. 57-8836). As an undercoat composition to coat polyphenylenesulfide resins on a metal substrate, a composition is known that includes a combination of metal powder consisting of aluminium powder and zinc powder, and silicate compounds such as colloidal silica, alkyl silicate, etc. (disclosed in Japanese Laid-Open Patent Publication No. 53-25649).
However, these inorganic binders, which have excellent heat-resistance, do not have sufficient adhesiveness to both the metal substrate and the upper resin layer. Thus, when the resulting coated metal substrate comes into contact with hot water, the coating layer blisters or peels.